1. Field of the Invention
This invention relates to an exchange coupling film utilizing the exchange coupling between an antiferromagnetic film and a ferromagnetic film and magnetoresistive elements such as magnetic field sensors and reproducing magnetic heads which are provided with such an exchange coupling film as mentioned above.
2. Description of the Related Art
The study of a magnetic head using a magnetoresistive element as a reproducing head for the sake of high-density magnetic recording has been heretofore studied. At present, a thin film of 80at % Ni - 20at % Fe alloy (otherwise known as "Permalloy") is generally used as a magnetoresistive material. As alternatives to this material, artificial lattice films such as of (Co/Cu)n which exhibits a giant magnetoresistive effect and spin valve films have been attracting attention in recent years.
Incidentally, since magnetoresistive films using these materials have magnetic domains, the Barkhausen noise originating by multi-domain activities poses a serious problem in the way to the commercialization the films. Various methods for imparting a single domain to the magnetoresistive films have been being studied from various angles. A method which resides in controlling in a specific direction the domains of a magnetoresistive film by utilizing the exchange coupling between a magnetoresistive film which is a ferromagnetic film and an antiferromagnetic film is counted among these methods. As the antiferromagnetic material used for exchange coupling, a .gamma.-FeMn alloy has been heretofore finding wide recognition (disclosed in U.S. Pat. No. 4,103,315 and U.S. Pat. No. 5,315,468, for example).
Further, the technique utilizing an exchange coupling between an antiferromagnetic film and a ferromagnetic film for pinning a magnetization of a magnetic film of a spin valve film has been disseminating in recent years. Also for this purpose, a .gamma.-FeMn alloy has been used as an antiferromagnetic material.
The .gamma.-FeMn alloy, however, is deficient in resistance to corrosion, particularly to the corrosion due to water, and is at a disadvantage in suffering the yield of elements being produced to be notably lowered and the force of exchange coupling with a magnetoresistive element to be deteriorated with time as by the corrosion during the course of fabrication of the magnetoresistive element and by the corrosion by the moisture in the air.
The antiferromagnetic film which is made of the .gamma.-FeMn alloy is problematic in respect that the force of exchange coupling with the ferromagnetic film is markedly altered by the temperature environment. Since the temperature of element parts of the magnetic head ultimately rises to the neighborhood of 80.degree. C. during the operation of the magnetic head, the blocking temperature at which the force of exchange coupling between the ferromagnetic film and the antiferromagnetic film is wholly lost ought to be as high as permissible. Since the blocking temperature of the .gamma.-FeMn alloy system is not higher than 200.degree. C., the antiferromagnetic film under discussion is at a disadvantage in exhibiting only inferior lasting reliability.
U.S. Pat. No. 4,103,315, for example, discloses cases of using antiferromagnetic films made of such .gamma.-Mn alloys as PtMn and RhMn other than the .gamma.-FeMn alloy and also cases of using antiferromagnetic films made of such oxides as NiO. The anti-ferromagnetic films made of such .gamma.-Mn alloys as PtMn and RhMn, however, exhibit no sufficient force of exchange coupling with a ferromagnetic film. In contrast, the antiferromagnetic films made of such oxides as NiO exhibit inferior thermal stability and exhibit unstable force of exchange coupling with a ferromagnetic film at such high temperatures as exceed about 100.degree. C. Further, such oxide systems as NiO exhibit high electric resistance, permit no direct derivation of an electrode therefrom, and entail the disadvantage of complicating the construction of an element.
U.S. Pat. No. 5,315,468 discloses an observation that when an antiferromagnetic film is formed of such a .theta.-Mn alloy as NiMn which has a face-centered tetragonal (fct) crystal structure, the force of exchange coupling between the antiferromag-netic film and a ferromagnetic film is not degraded even in a range of high temperatures.
The antiferromagnetic film of this nature, as deposited, exhibits a very small force of exchange coupling with a ferromagnetic film and, for the sake of acquiring a fully satisfactory force of coupling, must undergo a heat treatment at a high temperature in the neighborhood of 250.degree. C. by all means. When the antiferromagnetic film of this sort is adopted, therefore, the process of production is complicated to the extent of lowering the yield of production and degrading the reliability of the product.
Antiferromagnetic films have been heretofore used, as described above, for attaining exchange coupling with ferromagnetic films as by abating the Barkhausen noise inherent in magnetoresistive elements. The conventional antiferromagnetic films, however, suffer from deficiency in force of exchange coupling with a ferromagnetic film or in resistance to corrosion particularly at elevated temperatures and incur difficulty in producing exchange coupling films of fully satisfactory reliability with a high yield.